After school and university in South Africa, Giles moved to the UK in 1963. He studied Law at Magdalen College, Oxford (where he was a Senior Mackinnon Scholar), and obtained MA and BCL degrees. In 1966–67 he was a Member of the Faculty of Law at the University of California at Berkeley and obtained a Fulbright Award.
He joined Slaughter and May, the international law firm based in the City of London, and was elected a Partner in 1975 with a corporate and mergers and acquisitions practice, much of it international. He was elected as the Senior Partner of the firm in 1993 and served two terms of office, retiring in 2001. During his time at the firm he was extensively involved as legal adviser to HM Government on its programme of privatisations from 1983 to 1991, following which he was awarded the CBE.
Giles served on the Hampel Committee on Corporate Governance and the Financial Reporting Council and chaired the Law Group of the UK/China Forum. He held two non-executive directorships of public listed companies.
From 2001 to 2013 he was the Master of Pembroke College, Oxford. He chaired the Governing Body, Finance Committee, Strategic Planning Committee, Development Committee and New Build Steering Committee. During his time at Oxford, Giles served for two years as the Chair of the Conference of Colleges of Oxford University (the body on which all 38 colleges of the university are represented and which coordinates their policy and strategy on all university/college matters). He headed a comprehensive review on how the Conference was organised and operated and then led its implementation. Giles served for six years as a member of the Council of Oxford University (and of its Committees on Audit and Scrutiny, Planning and Resources and the Review of Donations). He was also, for 12 years, the Chair of the Nuffield Medical Trust. He was also a trustee of the Tanaka UK Japan Educational Foundation Trust and a participant in the Speakers for Schools programme.
Giles is a member of the UK Takeover Appeal Board and was appointed as Chairman of the Council in August 2019.